Assuming you have Windows XP first check that you have it set to get an IP address:

  • Right click the network icon bottom right of your screen
  • Click "status"
  • Click "properties"
  • Click "Internet Protocol (TCP/IP)"
  • Click "properties"
  • Select "obtain an IP address automatically"
  • Select "obtain a DNS server address automatically"
  • Click "OK"
  • Reboot PC

If that doesn't sort it then post back.